Re:..."... that moves freely but when put into forward that same arm moves about 15% and stops..."
Well.... most single lever helms have a "gear" mechanism that when meshed ( when you select combined throttle and shift operation) causes the "shift arm" to move the "shift cable". Sometimes this internal to the helm controller gear wears other bad stuff happens and it does not shift correctly. Normally this problem occurs over time, not suddenly as you indicated. However..... if you don't disconnect the shift cable at the drive per my previous post, you can't really tell if you have a helm control issue ( gear worn or broken) or if something is jammed in the drive. I can't fix this for you by remote control, all I can do is suggest a debug procedure that will help isolate the problem to: Bad helm control or improper connection at helm or issue in the drive link or issue in the drive....
Mechanics not wanting to work on it..... Most "marine mechanics" these days are Mercruiser only mechanics....or other branded product "mechanic". They are "trained" as Merc (or other) mechanics in the classic sense that training means... "when you see this, you do that"...where "do that" means "replace this part". Troubleshooting where you look at a problem and think your way to a solution is actual work and requires actual smarts and more than a little talent.... Rare these days.